Biomedical Engineering Reference
In-Depth Information
Pdx1 and Ngn3 to induce in vivo liver-to-pancreas reprogramming when using
adeno-associated viruses (AAV), but not when using plasmids together with an
irrelevant adenoviral (AV) vector. Because the capsid of adenoviruses (but not
AAV) is known to elicit an antigen-dependent immune response, these observa-
tions present yet another element that may be instrumental in inducing liver-to-
pancreas transdifferentiation.
8.2.1 Acinar Tissue Reprogramming
The exploitation of the liver as a potential source of new beta cells by means of
reprogramming was a fertile research field in the early 2000s. However, the
enthusiasm had started to plateau by the end of the decade after a plethora of
studies failed to capitalize on the promise that hepatocytes could be converted into
bona fide beta cells potentially usable for the clinical treatment of diabetes. In this
context, the recent reports on another source of reprogrammable tissue, the acinar
compartment of the pancreas, seem to have revitalized the field again.
Acinar exocrine cells are extraordinarily plastic, as evidenced by their ability to
turn into ductal cells [
89
,
90
], hepatic cells [
91
], and even beta-like cells [
92
-
95
],
the latter confirmed even by lineage-tracing techniques [
96
]. The combination of
simple growth factors such as leukemia inhibitory factor (LIF) and epidermal
growth factor (EGF) in vitro has been shown to reactivate Ngn3 expression, which
precedes the acquisition of a beta-like phenotype [
97
]. During embryonic devel-
opment, the pro-endocrine factor Ngn3 is activated in a Notch-dependent fashion.
Cells in which Notch signaling is inactivated down-regulate the Ngn3 repressor
HES-1, an event that unleashes Ngn3 expression and therefore endocrine cell
specification [
98
-
105
]. Based on this, Baeyens and colleagues [
106
] proposed that
down-regulation of Notch in terminally differentiated acinar cells in vitro could
also foster acinar to endocrine cell conversion, a hypothesis that was proven
correct in the above LIF ? EGF in vitro model.
A new wave of gene-based efforts at selectively converting acinar tissue to beta
cells was pioneered by Zhou and coworkers [
107
], who initiated their research by
conducting a systematic and combinatorial screening of 20 transcription factors
known to have a drastic effect in beta cell development [
108
,
109
]. The combi-
nation of Pdx1, Ngn3, and MafA turned out to be the only one conducive to in vivo
reprogramming when shuttled in adenoviruses and injected directly in the acinar-
rich pancreatic parenchyma. This is not surprising in view of earlier attempts to
transdifferentiate liver cells in which the use of Pdx1, MafA, and BETA2/NeuroD
(a downstream target of Ngn3 [
110
]) had proven superior to other combinations of
genes. Similarly, adenoviruses expressing the above genes also induced the
transdifferentiation of porcine neonatal pancreatic cell clusters [
111
].
Of note, in this set of experiments the investigators used Rag1-/- mice, a strain
typically used to minimize the occurrence of viral-induced immune responses such
as those earlier indicated by Wang and collaborators [
82
]. As early as 3 days after
